The Carpenter Bees Xylocopa varipuncta really are attracted to the Indigo Spires salvia blossoms.
Indigo Spires salvia produces nectar, and blooms nearly all year round here in the Bay Area.
I have only seen female carpenter bees in the garden, such as the one above, approaching an Indigo Spire whorl.
You can see that she is large, shiny and stout-bodied.
Note the short, dagger-like proboscis.
Here she is hanging upside down from one of the Indigo Spire blossoms, with wings in motion.